The present invention relates in general to the field of fiber optic sensors. In particular, the invention relates to a fiber optic interferometric sensor. More particularly, it relates to magnetic field and current sensing with a Sagnac loop, not having rotation sensitivity.
Over the past decade, Sagnac interferometers have received attention in the application of magnetic field sensing and current sensing. Fiber optic current sensors are particularly advantageous over iron-core current transformers, since fiber optic sensors are non-conductive and lightweight. Furthermore, fiber optic sensors also do not exhibit hysteresis and provide a much larger dynamic range and frequency response.
Fiber optic current sensors work on the principle of the Faraday effect. Current flowing in a wire induces a magnetic field that, through the Faraday effect, rotates the plane of polarization of the light traveling in the optical fiber wound around the current carrying wire. Faraday's law, stated as EQU I=H.multidot.dL
where I is the electrical current, H is the magnetic field and the integral is taken over a closed path around the current. If the sensing fiber is wound around the current carrying wire with an integral number of turns, and each point in the sensing fiber has a constant sensitivity to the magnetic field, then the rotation of the plane of polarization of the light in the fiber depends on the current being carried in the wire and is insensitive to all externally generated magnetic fields such as those caused by currents carried in nearby wires. The angle, .DELTA..phi., through which the plane of polarization of light rotates in the presence of a magnetic field is given by EQU .DELTA..phi.=V.intg.H.multidot.dL
where V is the Verdet constant of the fiber glass. The sensing optical fiber effectively performs the line integral of the magnetic field along its path, which is proportional to the current in the wire, when that path closes on itself. Thus we have that .DELTA..phi.=VNI where N is the number of turns of sensing fiber wound around the current carrying wire. The rotation of the state of polarization of the light due to the presence of an electrical current is measured by injecting light with a well defined linear polarization state into the sensing region, and then analyzing the polarization state of the light after it exits the sensing region.
A typical related-art optical fiber current sensor is disclosed in U.S. Pat. No. 5,644,397 issued Jul. 1, 1997 to inventor James N. Blake and entitled "Fiber Optic Interferometric Circuit and Magnet Field Sensor", which is incorporated herein by reference. Optical fiber current sensors are also disclosed in U.S. Pat. No. 5,696,858 issued Dec. 9, 1997 to inventor James N. Blake and entitled "Fiber Optics Apparatus and Method for Accurate Current Sensing", which is incorporated herein by reference.
The related-art sensing of current suffers from a number of difficulties. Exceptionally stable optical components are required for measuring the polarization state changes with the accuracy needed for certain applications such as revenue metering. In addition, birefringence present in the sensing region rotates the plane of polarization as well as current, yielding an indistinguishable signal. Mechanical disturbances such as vibrations in the sensing fiber can yield a time varying birefringence, which yields signals indistinguishable from time varying currents. Similarly, thermal disturbances may also produce erroneous results.
Accordingly, a need has arisen for a fiber optic current and magnetic field loop sensor that is insensitive to rotation and time-varying effects such as changing temperature and vibration, and does not require exceptionally stable optical components and low birefringence in the sensing region.